1. Field of the Invention
The invention pertains to safety control systems utilizing acoustic or electromagnetic waves to produce a control signal.
2. Description of the Related Art
It is known to use surveillance devices which actuate to produce a control signal when an elastomeric hose is deformed. For instance, devices for sensing pedestrian traffic, the entering of a vehicle into a service station, or the counting of traffic flow, may employ an elastomeric hose extending across the area to be surveyed whereby deformation of the hose compresses air therein to momentarily produce an increase in air pressure which is sensed to activate a control system, such as an alarm or counter. Such devices are susceptible to a wide variety of malfunctions.
Sensors employing deformable walls require a sudden and significant compression of the hose to produce a signal, and because the signal is very short, and possibly very weak, it is not uncommon for a signal to go undetected.
A very common surveillance area is a door or gate which may include pedestrian or vehicular traffic. For instance, a garage door opening provides passage for both vehicles and pedestrians, and garage doors are commonly operated by motor driven actuators. Because of the danger to a pedestrian, particularly a child, located within a garage doorway during door closing, a variety of safety devices have been proposed to prevent injury. For instance, pressure sensors may indicate the encountering of the door with an obstruction which produces an abnormal resistance to door movement which is sensed by the door actuator to reverse door movement. Also, it is known to utilize photoelectric devices within the doorway which prevent the door from closing in the event the light path of the photoelectric system is interrupted.
Previous safety systems utilizing pressure sensing, photoelectric light sensing, and the like, have a number of deficiencies such as those mentioned above, and further including alignment with respect to photocell devices and a high sensitivity to possible damage. Accordingly, there is a need for a more dependable sensing system for a surveillance area than is presently available.